Such a charge-pump circuit is known from the U.S. Pat. No. 4,847,519. The circuit known from this document comprises two current sources formed by two differential stages operating under cyclic control of up and down signals, a charge pulse being supplied at the output during a time interval in which only one of the two current sources is active. During the remainder of a cycle of the cyclic control signal the current sources are either inactive or simultaneously active, as a result of which no charge is supplied to the output of the circuit. The charge pulses at the output of the circuit are stored in a storage capacitance.
Although the known circuit arrangement has the advantage that it uses two current sources which can be very much alike and can therefore be paired in a suitable manner, it has the serious drawback of permanently consuming the total current of the two current sources owing to the use of the differential circuit.
With respect to the noise characteristics at the output of a charge-pump circuit of this type, a detailed analysis of the situation will show that the average noise is substantially proportional to the duty cycle of the control signals, the noise being substantially zero during the part of the cycle in which both current sources are inactive. To reduce the average noise at the output it is therefore advantageous to use control signals whose duty cycle is as small as possible. Moreover, if the current sources supply a large current, for example, several milliamperes or even some tens of milliamperes, this will improve the ability of a charge-pump circuit to rapidly change the value at the output in response to a sudden variation of the phase of an oscillator to be controlled.
Therefore, the prior-art charge-pump circuit is not suitable for uses in which the current consumption should be minimal, as in the case of battery-powered portable apparatuses.
It is an object of the invention to mitigate the above-mentioned problems. The invention features a charge-pump circuit whose power consumption is substantially smaller than that of the known circuit and whose noise at the output is reduced considerably.